This invention relates to ophthalmological instruments and, in particular, to magnifying ophthalmoscopes useful in surgery on the fundus or retina of a patient's eye.
A number of instruments are known for examination of the fundus of the eye. The direct ophthalmoscope is typically a hand-held instrument which illuminates but does not actually magnify the fundus. The image seen is erect rather than inverted, and the practitioner must position his or her eye in close proximity to the instrument and the patient. Moreover, the image has a narrow field of view and it lacks depth because it is focused with only one of the examining practitioner's own eyes. For these reasons, the direct ophtalmoscope is unsuitable for surgical use.
Another examining instrument is the indirect ophthalmoscope, which uses a magnifying lens, called a condensing or observation lens. This lens is placed in front of the patient's eye and is used to focus a magnified image of the fundus at an image plane located between the patient and the observer for stereo viewing. The condensing lens typically is used in conjunction with a binocular device, supported upon the practitioner's head, which reduces the practitioner's interpupilliary distance and provides illumination. However, the fundal image viewed by the practitioner is inverted and reversed, and this condition is unacceptable to the surgeon because of the need for precision and the chance of reflexive errors.
In an operating room, surgery on the fundus is typically performed with a flat contact lens sutured to the conjunctiva of the patient's eye. A slit lamp or surgical microscope positioned above the patient illuminates the fundus and focuses the fundal image transmitted by the patient's cornea and the contact lens. The contact lens is employed to cancel the strong refraction of the cornea, but can interfere with surgical activities. The contact lens also can cause edema of the corneal epithelium, as well as make detailed observation of the peripheral vitreous and retina more difficult or impossible. Moreover, the surgical microscope has disadvantages in use; the brightness of the fundal image decreases with increasing magnification. Insufficient brightness reduces the visibility, contrast and resolution of the image.
Another instrument, described as a stereo funduscope, is disclosed in U.S. Pat. No. 3,475,082 issued to Strietzel. This device operates with a condensing lens in manner similar to the indirect ophthalmoscopes described above, and employs a series of lenses to provide further magnification and to reverse the fundal images to an erect orientation in the eyepieces. A separating prism and a second prism switch the images transmitted to the left and right eyepieces in order to maintain proper stereopsis. This instrument is considered to suffer from significant operational constraints. As described in the patent, the distance between the condensing lens and the pupil of the patient's eye must be fixed by spacers resting against the patient's forehead. Further, the location of the condensing lens must correct for refractive errors of the patient's cornea so that the intermediate image of the fundus is focused precisely at the apex of the separating prism.
There accordingly exists a need for surgical ophthalmoscopes which can provide better magnification and good resolution, particularly to perform operations on the retina and around the macula of the eye. Such instruments desirably are to provide an erect view without sacrificing proper stereopsis and without sacrificing a substantial portion of the field of view; they hence are to maintain depth perception and enable viewing of nearly the entire fundus. Moreover, a surgical ophtalmoscope that can be operated without a contact lens and without spacers that might interfere with a surgeon's range of motion would satisfy a long-felt need.